This invention relates to a new nematic liquid crystal composition having positive dielectric anisotropy.
Nematic liquid crystal compounds or nematic liquid crystal compositions (hereinafter referred to as N-liquid crystal materials) are used for electro-optical display devices, utilizing the phenomenon that they are optically changed if an electric field is applied thereto. Electro-optical display devices with use of the N-liquid crystal material may be divided into two groups according to dieletric anisotropy of the N-liquid crystal material used. One group comprises devices in which there is employed a dynamic scattering mode (DSM), i.e. an optical scattering phenomenon caused by collision of an ion with a domain when an electric field is applied to a N-liquid crystal material having larger dielectric constant in a direction perpendicular to longitudinal axes of the molecules than dielectric constant in the longitudinal direction, i.e. nematic liquid crystal material having negative dielectric anisotropy (hereinafter referred to as Nn-liquid crystal material). Another group comprises devices in which there is employed an electric field effect mode (FEM), i.e. a change in lean or torsion of N-liquid crystal molecules caused by applying an electric field to an N-liquid crystal material having larger dielectric constant in a longitudinal direction of the molecules than that in a direction perpendicular to longitudinal axes of the molecules, i.e. nematic liquid crystal material having positive dielectric anisotropy (hereinafter referred to as Np-liquid crystal material). Many of the latter FEM-type devices are so-called "torsion effect-type display devices" (hereinafter referred to as TN-type display device) wherein a change in torsion of the molecules (one of the electric field effect modes) is utilized. The Np-liquid crystal composition of this invention is utilized for the TN-type display device.
As one of well-known Np-liquid crystal compounds, there are p-cyanophenyl-p'-n-alkylbenzoates having Formula I, ##STR3## wherein R.sub.1 is n-alkyl of 4 to 8 carbons atoms, which is disclosed in Japanese Application Kokai No. 48-96560. The compounds of Formula I are limited in a temperature range exhibiting nematic liquid crystal phase (hereinafter referred to as "mesomorphic range") as set forth in Table I, but Np-liquid crystal compositions having a broad mesomorphic range can be obtained by mixing two or more of these compounds. With reference to a Np-liquid crystal composition consisting of a mixture of two p-cyanophenyl-p'-n-alkylbenzoates, its phase diagram is shown in FIG. 1 and FIG. 2
Table 1 __________________________________________________________________________ Np-liquid crystal compounds Mesomorphic ranges __________________________________________________________________________ ##STR4## m.p. 67.degree. C 41.5.degree. C.about.m onotropic ##STR5## m.p. 60.5.degree. C 56.5.degree. C.about.monotropic ##STR6## 45.about.48.degree. C ##STR7## 44.about.56.5.degree. C ##STR8## 46.5.about.53.5.degree. C __________________________________________________________________________
A mixture of three p-cyanophenyl-p'-n-alkylbenzoates has in general a mesomorphic range broader than that of a mixture of two p-cyanophenyl-p'-n-alkylbenzoates. Among them, particularly a mixture in equal parts by weight of p-cyanophenyl-p'-n-butylbenzoate, p-cyanophenyl-p'-n-hexylbenzoate and p-cyanophenyl-p'-n-octylbenzoate has remarkably broad mesomorphic range, i.e. 7 to 47.5.degree. C.
The compounds of the Formula I and mixtures thereof are a Np-liquid crystal material with good chemical stability, but are limited in a mesomorphic range, particularly in a low temperature zone as compared with other Np-liquid crystal materials, for example p-alkylbenzylidene-p'-cyanoanilines of the formula, ##STR9## wherein R.sub.3 is ethyl, n-propyl, n-butyl, n-pentyl, i-hexyl, n-hexyl, n-heptyl and n-octyl, and mixtures thereof. For this reason, the Np-liquid crystal materials of Formula I are prevented from the use in the TN-type display device.